Nitrogen dioxide, NO2, is a traffic-related pollutant. Emissions are generally highest in urban rather than rural areas. Annual mean concentrations of nitrogen dioxide in urban areas are generally in the range 10-45 ppb, and lower in rural areas. Levels vary significantly throughout the day, with peaks generally occurring twice daily as a consequence of rush hour traffic. Concentrations can be as high as 200 ppb. Particulate matter is very fine and can be carried deep into the lungs where they can cause inflammation and a worsening of the condition of people with heart and lung disease. Further, the problem is not necessarily concentrated in the inner cities. Because many major road / motorway interchange complexes are situated in semi-rural areas, under conditions of near-stationary traffic, a rapid build-up of engine exhaust pollution can occur, which if the low-level atmospheric conditions are correct, will not be dispersed. Note: This image is of Nitrogen Dioxide, NO2, on October 16, 2004. Data Collected: 2004/09/24 - 2004/11/7. Animator: Lori Perkins (NASA/GSFC). Scientist: Mark Schoeberl (NASA/GSFC), Ernest Hilsenrath (NASA/GSFC), Pepijn Veefkind (Royal Netherlands Meteorological Institute KNMI). Platforms/Sensors/Data Sets: Aura/OMI, DMSP/OLS/Earth at Night.